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SubjectRe: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for superblock creation [ver #9]
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Andy Lutomirski <luto@amacapital.net> wrote:

> > Also you can't currently directly create a bind mount from userspace as you
> > can only bind from another path point - which you may not be able to access
> > (either by permission failure or because it's not in your mount namespace).
> >
>
> Are you trying to preserve the magic bind semantics with the new API?

No, I'm pointing out that you can't emulate this by doing a bind mount from
userspace if you can't access the thing you're binding from.

Now, we could create a syscall that just picks up an extant superblock using a
device and attaches it to a mount for you, but that would have to be at least
partially parameterised - which would be very fs-dependent - so that it can
know whether or not you're allowed to create another mount to that sb.

What you're talking about is emulating sget() in userspace - when we have to
do it in the kernel anyway if we still offer mount(2).

David
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